Since we established in the current granting period that inhalation of benzene can cause a significant reduction in CFC levels in the bone marrow one day later, we would like to determine the cell kinetics of this early reduction. The aims for the next granting period will be to answer the following questions: 1) What is the earliest time interval in which one can detect the reduction of femoral hemic progenitors (CFC)? For instance, can the reduction be seen as early as 1 or 2 hours after inhalation? 2) Will dose scheduling of benzene affect the CFC levels? 3) Of single vs. multiple doses of benzene inhalation, which is more toxic to hemic CFC? 4) What is the sensitivity of splenic CFU (stem cell) to benzene as compared to CFC? How does Dicke's in vitro assay for CFU compare with the in vivo splenic CFU in regard to benzene sensitivity? 5) What other CFC subsets of the immunopoietic system, e.g., lymphocytic or erythrocytic, are sensitive to benzene inhalation?